A major impasse to investigating the control of microtubule properties and functions has been the lack of native tubulins derived from microtubules of defined characteristics and in sufficient quantities for biochemical studies. Outer doublet microtubules represent a model system for an investigation of this type, since the A-and B-subfibres differ not only from each other in morphology, function and chemical composition of their constituent tubulins, but also from brain microtubules. Until recently, it was not possible to exploit this system since tubulin solubilized from outer doublets was denatured. Assembly-competent tubulin has now been obtained from outer doublets and we propose to utilize this system to examine the control of microtubule properties and functions. Our approach relies on methodology developed for brain microtubules and which has been used to show that tubulin addition and loss from microtubules occurs at opposite ends of the microtubules, resulting in a flow of tubulin subunits through the microtubules. The importance of this concept is that it allows us to monitor factors influencing the tubulin flow rate and thereby gain insight into the control of microtubule properties and function in vivo. We propose to approach this problem in two stages. First, we plan to characterize the microtubules reassembled in vitro from outer doublet tubulin with respect to their opposite end assembly-disassembly parameters and exploit differences with vertebrate brain microtubules. Secondly, we plan to utilize the characterized assembly system as an assay method for examining the effect of chemically distinct tubulin populations (derived from A- and B-subfibre microtubules) and axonemal proteins on the parameters of the assay system. In this way we hope to gain insight into the functional importance of tubulin chemistry for assembled microtubules, and the revelance of associated proteins for microtubule properties and function.